Differentially coherent detection of certain kinds of digitally modulated RF waveforms (e.g., 2, 4, or n level phase shift keying) is a relatively simple method of recovering the digital data. With reference to FIG. 1, a commonly used technique for differential coherent detection of a quartenary phase shift keyed (QPSK) signal is shown. The QPSK signal is applied to a channel filter 10 which provides an output to the differential detector. This filtered signal is applied on the one hand to a first mixer 14 and to a second mixer 16 via delay device 12. The output of the delay device is shifted by -.pi./2 radians in phase shifter 18 and applied to the second input of first mixer 14, while the undelayed filtered signal is applied to a second input of second mixer 16. The output of second mixer 16 is applied to a well known decision device 20 which detects the value of the in-phase bits, while the output of mixer 14 is applied to decision means 22 which detects the quadrature bit patterns.
Thus, the technique illustrated in FIG. 1 is performed directly at RF, thereby eliminating the requirement for local oscillators and mixers to thereby realize a particularly simple device.
However, in actually practicing the technique shown in FIG. 1, it has been found that problems occur due to the temperature dependence of the delay device 12. That is, the signalling interval which determines the required delay time is usually of sufficient length that there are many 360 degree phase shifts of the RF waveform stored in the delay element. As a result, the phase shift as a function of temperature will be large, leading to an unacceptable degradation in the detection process.
One such solution for this problem is described in U.S. Patent Application Ser. No. 143,682 to Lee, and in "Temperature Compensated BaTi.sub.4 O.sub.9 Microstrip Delay Line", by Y. S. Lee and W. H. Childs, 1979 International Microwave Symposium Digest,pp. 419-421. Minor refinements of the above referenced techniques are described in "On the Design of Temperature Stabilized Delay Lines", by P. DeSantis, in IEEE Transactions on Microwave Theory and Techniques, MTT-28, September 1980, pp 1028-1029.
The above referenced techniques for solving the problems associated with temperature dependent delay elements are illustrated in functional form in FIG. 2. The intent of this approach is to absolutely stabilize the phase variation as a function of temperature across the delay element 24. This is accomplished by using two different substrate materials with opposite signs of temperature coefficients. As an example, one of the materials disclosed as having a negative temperature coefficient is barium tetratitanate (BaTi.sub.4 O.sub.9) ceramic, and the substrate material having the positive temperature coefficient is saphire. Using microwave integrated circuitry technology, simple transmission lines were constructed using these substrate materials. The line lengths are chosen in such a way that the required time delay and a temperature stable phase characteristic are achieved, as explained in the references.
However, materials with negative temperature coefficients are unusual in nature, and the designer is thus strictly limited in the available choices of negative temperature coefficient materials. In particular, the above mentioned article by Lee and Childs indicates that a 1.5 dB loss per nanosecond of delay is experienced at 24 GHz using the barium tetratitanate ceramic microstrip line. A typical 48 MBs QPSK system would require 42 ns of delay, thereby imparting a 60 dB loss in the delayed path. Although this loss may be somewhat reduced, excessive loss appears to be a major obstacle in applying the abovementioned technique to systems having bit rates in this range.